Let's say you just found a bug in the current nightly build and you know that bug
didn't exist in the nightly from a few months ago. You want to find the regression
range of this bug (the window of commits the bug was introduced in).

To find the range run the mozregression command on the command-line
(if you're on Windows this is Mozilla Build command-line, if you're on Mac or Linux
this is the Terminal) and give it a known "good" date with the good argument:

mozregression --good=2010-03-16

You can also specify the most distant known "bad" nightly date with the bad argument.

mozregression --good=2010-03-16 --bad=2010-09-08

This will guide you through a bisection, automatically downloading and opening
nightly builds from various dates (on new, clean profiles) and asking you whether the bug exists in them.
After doing this a few times you'll get the regression range, something like this:

Last good nightly: 2010-09-08 First bad nightly: 2010-09-09
Pushlog: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=36f5cf6b2d42&tochange=8e0fce7d5b49

There's also a sister script, moznightly that will just download and run a nightly from a particular date: